On Rahman gas
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Why is it said that feeding expanded air to the upper tray can reduce distillation energy consumption?
In terms of air separation, the goal is to achieve separation; the lower the pressure, the less energy is required.
Under normal operating conditions, the amount of expanded air fed into the upper tower is limited; this limitation is determined by the minimum reflux ratio in the distillation section of the upper tower, in order to maintain the purity of the product gas. Increasing the amount of expanded air fed into the upper tower appropriately can help improve the yield of the product gas, while simultaneously reducing energy consumption；
Allowing the expanded air to enter the upper tower cleverly combines refrigeration and air distillation together; the air used for refrigeration is no longer discharged but instead enters the upper tower to participate in the distillation process, which naturally reduces the energy consumption associated with distillation. Of course, this was in the year 1932 at that time. Just as we talk about booster turbine expanders these days, in the past expanders were equipped with air machines for braking, which were noisy and not economical; later on, expanders were fitted with generators to recover energy, and it was only then that booster turbine expanders came into use.
It is said that by introducing expanded air into the upper tower, more oxygen can be recovered; in other words, the amount of oxygen increases. With the same volume of air processed, more oxygen is produced, so surely energy consumption should decrease, right? However, this increase in oxygen level is limited. As I’m sure everyone knows, if too much expanded air is fed into the upper column, exceeding its distillation capacity, then the oxygen level will decrease as well, which in turn leads to unstable operating conditions in the argon column. So there must be a limit!